336 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



The leading position in gametophyte research after Strasburger was taken 

 by Coulter and Chamberlain (1917) at the head of the Chicago school of botan- 

 ists. Their results have had a great share in modifying our ideas of the phy- 

 logeny of the several groups. Summaries were also published by Schnarf (1933) 

 and Chamberlain ( 1935 ) . It was established that in the male gametophyte there 

 has been a tendency to reduce the number of cell divisions. Types of pollen in 

 which prothallial cells are formed are considered more primitive than those 

 where such cells do not occur. In cycads, as well as in Ephedra and probably 

 also Welwitschia, there is only one prothallial cell. Gnetum appears to have 

 only a prothallial nucleus, but in Ginkgo and the Pinaceae there are two of these 

 cells, while in most of the remaining conifers and in the Taxaceae they are 

 eliminated. The Araucariaceae and Podocarpaceae generally have two prothal- 

 lial cells in the mature pollen grain, but these sometimes divide secondarily to 

 form many-celled tissue. Only two male gametes are as a rule produced in the 

 pollen tubes. A noteworthy exception was discovered by Caldwell in Micro- 

 cycas, where 8-10 spermatogenous cells are always formed instead of one, and 

 the number of spermatozoids is increased correspondingly. The stalk cell formed 

 by the division of the antheridial cell has apparently retained its original func- 

 tion as a spermatogenous cell and its capacity to divide. The two male gametes 

 of the higher gymnosperms are either highly organized cells, or have lost their 

 cell walls. The taxads show reduction in the direction of the elimination of 

 one of the two gametes. 



The development of the female gametophyte has been found to be marked 

 by a period of free nuclear divisions followed by a period of wall formation. 

 Welwitschia and Gnetum differ in a remarkable manner from all other gymno- 

 sperms. In the typical gymnosperms the main feature is the reduction of the 

 archegonium. The differentiation of a definite ventral canal cell as in Ginkgo 

 and the Pinaceae is a primitive feature. In the majority of the cycads and coni- 

 fers, wall formation has been entirely eliminated between the egg and ventral 

 canal nucleus. A neck canal cell is not found; the two-celled neck, which occurs 

 in the cycads and Ginkgo, is probably primitive, while the many-celled neck of 

 Ephedra appears to be an advanced feature. The numbers and arrangement 

 of the archegonia vary considerably. The condition in most cycads, where a 

 limited number of archegonia occur free from one another in the micropylar 

 end of the gametophyte, is looked upon as relatively primitive, while the spread- 

 ing of the archegonia over a larger part of the gametophyte and the forma- 

 tion of archegonial complexes — which both occur in the Taxodiaceae and Cu- 

 pressaceae— are considered advanced features. 



Knowledge of the external structure of the megaspores and microspores was 

 also furthered. According to Thomson (1905) a megaspore membrane is pres- 

 ent in all gymnosperm groups. From the point of view of the relative develop- 

 ment of this coat and the tapetum he concluded that the Pinaceae, as well as 

 some forms of the Taxodiaceae and Podocarpaceae, form ancient groups, and 

 the taxads the most recent group. The number and arrangement of the places 

 of exit on the surface of the pollen grains were investigated by Pohl (1928) 

 and Tammes (1930). The basic type in the gymnosperms is furnished with a 

 single longitudinal fold or germinal furrow at the distal pole, and occurs in the 

 cycads, Ginkgo, Pinaceae, and Podocarpus. When two air sacs occur, they are 



